Neutron-flux responsive switch

ABSTRACT

The present invention is a neutron-activated switch having a neutron-flux sensitive disk soldered to a spring steel strip. The spring steel strip has an end attached to one electrical contact while the other soldered end is bent away from a second contact. A neutron burst impinging on the neutron-sensitive disk generates heat which melts the solder thereby allowing the switch to close. The switch has application in circuits used in specimen crack detection systems.

United States Patent [191 Parker NEUTRON-FLUX 'RESPONSIVE SWITCH [75] Inventor: Robert Parker, Danville, Calif.

[73] Assignee: The United States of America as represented by the United States Atomic Energy Commission, Washington, DC.

[22] Filed: Sept. 6, 1972 [21] Appl. N0.: 286,787

52 user 250/390, 250/392 51 1m. 01. ..G01t3/00 5s FieldofSearch 1.250/s3.1,390,391,

[56] References Cited UNITEDYSTATES PATENTS Zinn 250/8311 X Jan. 1, 1974 LeClerc 250/83.1

3,225,196 Gigon et al. 250/83.1

Primary ExaminerArchie R. Borchelt Attorney-John A. Horan [57] ABSTRACT The present invention is a neutron-activated switch having a neutron-flux sensitive disk soldered to a spring steel strip. The spring steel strip has an end attached to one electrical contact while the other soldered end is bent away from .a second contact. A neutron burst impinging on the neutron-sensitive disk generates heat which melts the solder thereby allowing the switch to close. The switch has application in circuits used in specimen crack detection systems.

3 Claims, 3 Drawing Figures PATENTEBJAN 1 19M I IIIIIIII TEST SPECIMEN FIG. 2

NEUTRON-FLUX RESPONSIVE SWITCH BACKGROUND OF THE INVENTION There exists a need for an inexpensive passive crack detector system to determine how soon after exposure to radiation a mechanical component cracks. In many cases, material failure is preceded by the formation of hairline cracks in the specimen. The exact time of the crack formation is important because it enables the designer to calculate the total energy absorbed by the specimen up to the point of incipient failure. In the case of an electrically conductive specimen, the time of crack formation can be determined by circulating a current through the specimen and noting the time when the current abruptly decreases due to the increased electrical resistance caused by crack formation. If the specimen is not electrically conductive, such as a ceramic, a thin metal film can be deposited on its surface.

A system has been designed so that a neutron-flux responsive switch is activated by the impinging radiation. The switch allows current from cadmium or other batteries to flow through a conductive element on the surface of the component; An interruption of the current by the formation of a crack is recorded on a passive current integrating device in series with the conductive element. The records may be visually inspected to determine when any cracks occurred. The crack detector system described hereinbelow occupies only a few cubic inches of space and operates reliably in severe thermal shock and radiation environments.

SUMMARY OF THE INVENTION The present invention comprises a neutron-activated switch in series with a battery and a current integrating device. These elements are in series with a conductive strip which completes the circuit back to the switch. The switch closes in less than I msec. after exposure to neutron flux. Current from the battery then flows through the integrating device. If a crack occurs at any time between and 100 msec., the current will cease and the time the crack occurred will be recorded on the integrating device.

It is an object of the invention to provide a unique neutron-flux responsive switch. It is a further object to provide a system where such a neutron-flux responsive switch is incorporated into a circuit having a test specimen subject to impinging radiation and a readout means which indicates the length of time the switch is closed. A further object is to provide a simple selfcontained system for measuring the time of crack formation in material test specimens where the test specimens are exposed to neutron flux.

BRIEF DESCRIPTION OF THE DRAWINGS DESCRIPTION OF THE PREFERRED EMBODIMENT FIGS. 1 and 2 depict the neutron-activated switch 8 wherein a strip of spring steel 10 or other leaf spring is fixed at one end by means of glue or epoxy bond to an electrical contact plate 12. The other end of the steel strip 10 is bent away from a second contact plate 14 by being soldered at 16 to an alloy disk 18 thus forming a gap. The disk 18 can be made of any alloy containing a high concentration of fissionable: material, for example, an alloy containing at least percent U 235. A typical alloy disk is one-quarter in. wide, one-half in. long, and 50-60 mil thick. The solder 16 may be any low temperature solder such as 60/40 tin and lead. The disk 18 is seated in a nonconducting base 20 which may be made of any plastic insulating material. A spacer 22 is interposed between the disk 18 and the contact plate 14 next to the soldered end of the :strip 10. A top plate 24 also of a plastic material, is fastened to the lower base 20 by means of flat head machine screws 26. Electrical leads 28 are attached to contact plates 12 and 14.

When the switch is exposed to radiation such as a neutron-burst, neutrons impinging on the switch instantaneously heat the disk 18 above the melting point of the solder. The melted solder releases the free end of the strip 10 which swings across the narrow gap to complete the circuit between the first and second electrical contacts. By proper design of the component parts, switch closure time can be made less than one millisec- This switch may be used for any application which requires a fast-closing switch activated by neutrons. The minimum required total integrated flux to cause melting of the solder is 10 neutrons/cm? It is very well suited for use with a specimen crack detection system as shown in FIG. 3. The system consists of the neutronactivated switch 8 connected in series with an electrically-conductive test specimen 30. The circuit contains a battery 32 and a current integrator 34 in series with the switch. The battery 32 may be a nickel cadmium rechargeable type. The current integrator may be any convenient type.

The operation of the crack detection system is as follows, assuming that a test specimen is exposed to a neutron burst. The neutron sensitive switch would close after the exposure and permit current to flow around the circuit loop. The constant current from the battery would flow through the test specimen, through the integrator, and back to the battery. Since the battery current is constant, the integrator reads out a linear function of time, i.e., the integrator serves as a clock which is started by closure of the switch. Thus, if the test specimen should crack after closure of the switch, thereby increasing electrical resistance in the circuit loop, the integrator would indicate the exact time of crack formation.

Another variation of the crack detector system might include a separate parallel switch containing circuits connected to the test specimen having an identical design to the one shown in FIG. 3 but where the switch would be sensitive to acceleration or to X-rays. Each circuit loop in this variation (not shown) would contain a battery and a current integrator in series with the switch. The batteries would have the same voltage and polarity connections to minimize interaction among the multiple parallel circuit loops.

It should be understood that the neutron sensitive switch is not restricted for use only with the crack detection system described above, but can be used in other systems where it is advantageous and in other fields of endeavor. Various changes and modifications can be made without departing from the scope of the invention.

sponding to the interruption of current flow.

2. The crack detector system of claim 1 in which the test component is a ceramic having a thin metal film deposited on its surface.

3. A crack detecting circuit for conductive test specimens as described in Claim 1 wherein the neutron-flux responsive switch has a leaf spring member which is soldered to an open position forming a gap, said spring member closing when thesolder melts due to the heat generated by a neutron sensitive disk by said impinging radiation thus freeing the leaf spring end to close the gap. 

1. A crack detector system for conductive test components exposed to neutron flux comprising: an electrically conductive test specimen, a neutron-flux responsive switch, a source of direct current, and a current integrating readout means serving as a clock to register the time of crack formation, the neutron-flux responsive switch being normally open and responding to impinging radiation by closing thus completing the crack detecting circuit, thereby permitting the current integrating means to record the amount of current flowing and to record the time of crack formation in the specimen corresponding to the interruption of current flow.
 2. The crack detector system of claim 1 in which the test component is a ceramic having a thin metal film deposited on its surface.
 3. A crack detecting circuit for conductive test specimens as described in Claim 1 wherein the neutron-flux responsive switch has a leaf spring member which is soldered to an open position forming a gap, said spring member closing when the solder melts due to the heat generated by a neutron sensitive disk by said impinging radiation thus freeing the leaf spring end to close the gap. 